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Organometallics 2009, 28, 2300–2310
C-H Activation of Imines by Trimethylphosphine-Supported Iron Complexes and Their Reactivities Sebnem Camadanli,*,† Robert Beck,† Ulrich Flo¨rke,‡ and Hans-Friedrich Klein† Eduard-Zintl-Institut fu¨r Anorganische and Physikalische Chemie, Technische UniVersita¨t Darmstadt, Petersenstrasse 18, Darmstadt, 64287, Germany, and Anorganische und Analytische Chemie, UniVersita¨t Paderborn, Warburger Strasse 100, Paderborn, 33098, Germany ReceiVed August 25, 2008
Introduction The direct and catalytic transformation of hydrocarbons to various useful chemicals via C-H bond activation is of considerable interest to chemical industries and remains a challenge to chemists due to the inherent difficulty associated with breaking such robust bonds.1 The introduction of transition metals into this area has given new opportunities, and there has been a massive effort to achieve selective C-H bond activation by transition metal complexes.2 Activation of aromatic C-H bonds by ortho-metalation is a promising example of synthetically applicable catalytic systems.3 In most cases, the direct use of aromatic compounds in synthesis relies on the presence of a more reactive group C-X (X ) Cl, Br, I) than a C-H bond.4 This chemistry can be very successful, but the manufacture of aryl halides is not an environmentally friendly process. Furthermore, these reactions typically produce halide salts as byproducts. Therefore, the bulk synthesis of aryl derivatives proceeding by such C-H bond activation requires the development of practical catalytic ways to directly activate C-H bonds of arenes. Iron-based catalysts of comparable activity would be desirable owing to their potentially lower cost and lack of toxicity and environmental impact. Despite its advantages, iron was relatively underrepresented in the field of catalysis compared to the other transition metals.5 Even though strong Lewis acid FeCl3-catalyzed Friedel-Crafts reactions of electron-rich arenes are a well-known process for the formation of new C-C bonds from aromatic C-H bonds, rearrangements can occur to produce a more highly substituted product.6 There are some well-known iron-based catalysts, for example, the * To whom correspondence should be addressed. E-mail:
[email protected]. † Technische Universita¨t Darmstadt. ‡ Universita¨t Paderborn. (1) (a) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083. (b) Kakiuchi, F.; Chatani, N. AdV. Synth. Catal. 2003, 345, 1077. (2) (a) Shilov, E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879. (b) Kakiuchi, F.; Murai, S. ActiVation of UnreactiVe Bonds and Organic Synthesis; Springer: New York, 1999. (c) Dyker, G. Angew. Chem., Int. Ed. 1999, 38, 1698. (d) Fujiwara, Y.; Takaki, K.; Taniguchi, Y. Synlett 1996, 591. (e) Gupta, M.; Hagen, C.; Kaska, W. C.; Cramer, R. E.; Jensen, C. M. J. Am. Chem. Soc. 1997, 119, 840. (f) Reis, P. M.; Silva, J. A. L.; da Silva, J. J. R. F.; Pomberio, A. J. L. Chem. Commun. 2000, 1845. (3) (a) Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529. (b) Lenges, C. P.; Brookhart, M. J. Am. Chem. Soc. 1999, 121, 6616. (c) Matsumoto, T.; Taube, D. J.; Periana, R. A.; Taube, H.; Yoshida, H. J. Am. Chem. Soc. 2000, 122, 7414. (4) (a) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009. (b) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2000, 122, 4020. (5) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. ReV. 2004, 104, 6217. (6) (a) Bandini, M.; Melloni, A.; Umani-Ronchi, A. Angew. Chem., Int. Ed. 2004, 43, 550. (b) Wang, Z.; Sun, X.; Wu, J. Tetrahedron 2008, 64, 5013.
Chart 1
landmark discovery of highly active bis(imino)pyridine iron complexes for ethylene polymerization and R-olefin oligomerization reported by Gibson,7 DuPont,8 and Brookhart.9 Iron dihalide precatalysts are noteworthy in their ability to oligomerize R-olefins.10 Besides that, there are some very recently reported iron catalysts with N-donor functionality, for example, iron catalyst for the asymmetric hydrogenation of polar bonds by Morris et al.,11 iron catalysts for olefin hydrogenation by Chirik et al.,12 and the achiral iron catalyst by Casey and Guan for ketone and imine hydrogenation.13 Syntheses with catalytic amounts of ruthenium or rhodium, preferentially using an N donor as anchoring group in a substrate molecule, when followed by a coordination of a suitable cosubstrate and elimination of the metal, have become an indispensable method of C,C-coupling in organic chemistry.14 Surprisingly, exploring the coordination chemistry of the postulated [N,C]-cyclometalated intermediates has remained a great challenge to date.15 Using complexes of low-valent iron (1, 2, and 4) and cobalt, we have recently conducted cyclometalation reactions that mimic the first steps in the proposed catalytic cycle (Chart 1).16 In this paper, the activation and functionalization of C-H bonds by solution-phase iron metal-based systems are presented, with an emphasis on the activation of aromatic C-H bonds. We herein report the synthesis and structural analyses of a series (7) Britovsek, G. J. P.; Gibson, V. C.; Kimberely, B. S.; Maddox, P. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem. Commun. 1998, 849. (8) Bennett, A. M. A. (DuPont) World Patent Application 98/27124, 1998. (9) Small, B. L; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 102, 4049. (10) Gibson, V. V.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283. (11) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew. Chem., Int. Ed. 2008, 47, 940. (12) (a) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794. (b) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organometallics 2005, 24, 5518. (13) Casey, C. P.; Guan, H. J. Am. Chem. Soc. 2007, 129, 5816. (14) Kakiuchi, F.; Murai, S. Acc. Chem. ReV. 2002, 35, 826. (15) Coalter, J. N., III; Streib, W. E.; Caulton, K. G. Inorg. Chem. 2000, 39, 3749. (16) Klein, H.-F.; Camadanli, S.; Beck, R.; Leukel, D.; Flo¨rke, U. Angew. Chem., Int. Ed. 2005, 44, 975.
10.1021/om800828j CCC: $40.75 2009 American Chemical Society Publication on Web 03/12/2009
C-H ActiVation of Imines by Iron Complexes Scheme 1
of ortho-metalated hydrido-iron and methyl-iron complexes and their reactivities.
Results and Discussion 1. Reactions of Fe(PMe3)4 with Ketimines, PhRCdNH (R ) C6H5, C(CH3)3). The M-H bond plays an important role in organometallic chemistry because metal hydrides can undergo insertion with a wide variety of unsaturated compounds to give stable species or reaction intermediates containing M-C bonds.17 Not only are these synthetically useful, but many of the catalytic reactions involve hydride insertion as the key step.18 Fe(PMe3)4 upon reaction with ketimines were transformed to Fe-H complexes under mild reaction conditions. Stoichiometric amounts of diphenylketimine and tert-butylphenylketimine reacted with Fe(PMe3)4 in pentane at -70 °C. During warmup to room temperature, the color of the mixtures changed from dark brown to violet for 1 and to reddish violet for 2 (Scheme 1).16 The reaction likely begins with the coordination of the iron adduct to the nitrogen or CdN bond by substituting one of the trimethylphosphines, which brings the metal closer to the ortho C-H bond (Scheme 1). This chelation assistance results in an easier and highly selective C-H bond cleavage to give the cyclometalated iron(II) complex. Compounds 1 and 2 crystallize from pentane at -27 °C as dark violet prisms. While compound 2 is moderately sensitive, compound 1 is extremely hygroscopic. Under 1 bar of argon, the solids decompose at 105-118 °C. IR spectra of the compounds give evidence for hydrido-iron(II) complexes with ν(Fe-H) stretching frequencies at 1730 cm-1 for 1 and 1795 cm-1 for 2. ν(N-H) bands are observed with bathochromic shifts between 45 and 70 cm-1, indicating coordination through the N atom. In 1H NMR spectra, Fe-H resonances appear at -16.4 ppm for 1 and -17.5 ppm for 2 as doublets of triplets due to the coupling of the hydride nucleus with trans- and cisdisposed trimethylphosphines.19 Both compounds show two sets of anisochronic PMe3 groups, which indicate that the iron centers have an octahedral coordination environment made up of three meridional PMe3 ligands (Figure 1). The presence of the orthometalated imine complexes is also supported by the 13C{1H} NMR spectra. Compound 1 contains a doublet at 180.2 ppm (JP,C ) 10.3 Hz) due to a coupling of the imine carbon with one of the P nuclei and a multiplet at 204.1 ppm assigned to the metalated aromatic carbon. In compound 2, these resonate as multiplets at 186.1 ppm (CdN) and 200.1 ppm (Fe-C). The 31 P{1H} NMR spectra of the two compounds, which are temperature invariant, show two sets of PMe3 groups with couplings of 37 Hz. The PMe3 group trans to the metalated (17) Dedieu, A. Transition Metal Hydrides; VCH: New York, 1992. (18) (a) Masters, C. Homogenous Catalysis; Chapman Hall: London, 1981. (b) Parshall, G. W. Homogenous Catalysis, Wiley-Interscience: New York, 1980. (c) Jordan, R. B. Reaction Mechanisms of Inorganic and Organometallic Systems; Oxford University Press: Oxford, 1991. (19) Allen, R. O.; Dalgarno, S. J.; Field, L. D. Organometallics 2008, 27, 3328.
Organometallics, Vol. 28, No. 7, 2009 2301
carbon gives a triplet with a resonance at around 23 ppm, and the two isochronic trans phosphines appear at 18 ppm as doublets in both cases. The molecular structure of 1 shows a slightly distorted octahedral frame of donor atoms centered by an iron atom that bears three meridional PMe3 donor groups (Figure 2). C, N, and H atoms of the metalacycle occupy three remaining ligand positions. The angle P3-Fe1-P1 ) 152.878(18) Å shows a significant deviation from 180° because of spatial relaxation. The ortho-metalated imine ligand acts with a bite angle of 79.0(6)°, and the sum of internal angle for the five-membered metalacycle (539.90°) approach the ideal value for a planar fivemembered ring (540°). The Fe-P distances are in the typical range of 2.18-2.22 Å.20 The Fe1-P2 distance is lengthened due to the trans influence of the carbon donor, while N1-Fe1 and C1-Fe1 distances are also in typical ranges.21 The CdN bond is lengthened upon coordination from 1.237(3) Å to 1.314(2) Å.22 The Fe1-H1 1.480(19) Å is in the typical range for molecular iron hydrides.23 A view of the molecular geometry of 2 is shown in Figure 3. The X-ray diffraction study on a single crystal of 2 confirms the structure proposed in Scheme 1 and shows the same structural characteristics as 1. 2. Reactions of Fe(CH3)2(PMe3)4 with Diphenylketimine (Ph2CdNH). The possibility of direct introduction of a new C-C bond via direct C-H bond transformation is a highly attractive synthetic strategy in synthesis owing to the ubiquitous nature of C-H bonds in organic substances, where the range of substrates is virtually unlimited. The activation of C-H bonds and the formation of C-C bonds in a single preparative step combine economy, efficiency, and elegance. The reaction of diphenylketimine with Fe(CH3)2(PMe3)4 afforded a methyliron(II) complex with a C,C-coupling between an sp2 carbon of the aromatic backbone and an sp3 carbon of the CH3 fragment (Scheme 2). The C,C-coupling in this reaction suggests an unusual reaction pathway. Fe(CH3)2(PMe3)4 reacts first with diphenylketimine to produce a methyl-iron(II) intermediate by elimination of methane. C,C-coupling between methyl and phenyl groups and reductive elimination afford the new aromatic backbone. The Fe(0) complex in solution reinserts into the ortho C-H bond of the unsubstituted phenyl ring and forms the hydrido-Fe(II) complex 3. IR and NMR data support the structure assigned for 3. In the IR spectrum, the absorption bands at 3279 and 1761 cm-1, respectively, represent ν(N-H) and ν(Fe-H) vibrations (Table 1 summarizes the properties of complexes 1-3). In the 1H NMR spectrum the new CH3 group formed by C,C-coupling appears as a singlet at 2.21 ppm. The Fe-H group resonates at -17.2 ppm as a doublet of triplets. In 13C{1H} NMR the Ar-CH3 carbon appears as a singlet at 21.1 ppm. The imine carbon atom resonates at 181.2 ppm as a doublet (3JP,C ) 9.8 Hz), and the metalated carbon gives a multiplet at 203.1 ppm. In the 31P{1H} NMR the trans PMe3 groups resonate as a multiplet at 15.7 ppm and PMe3 cis disposed to H appears as a triplet with a coupling constant 2JP,P of 38.3 Hz. These data support a meroctahedral hydrido-Fe(II) complex with three PMe3 ligands and a coordinated imine backbone. (20) Fru¨hauf, H.-W.; Wolmersha¨user, G. Chem. Ber. 1982, 115, 1070. (21) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006, 45, 7252. (22) Archer, A. M.; Bouwkamp, M. W.; Cortez, M. P.; Lobkovsky, E.; Chirik, P. J. Organometallics 2006, 18, 4269. (23) (a) Bau, R.; Yuan, H. S. H.; Baker, M. V.; Field, L. Inorg. Chim. Acta 1986, 114, L27. (b) Baker, M. V.; Field, L. Appl. Organomet. Chem. 1990, 4 (5), 543.
2302 Organometallics, Vol. 28, No. 7, 2009
Camadanli et al.
Figure 1. 1H NMR spectra of 2 (500 MHz, d8-THF, 300 K).
Figure 2. Molecular structure of 1 (ORTEP plot with hydrogen atoms, FeH position indicated).
X-ray diffraction analysis of 3 indicates that the imine reactant binds to iron through the N1 and C1 atoms and forms a fivemembered metallacycle with a bite angle N1-Fe1-C1 of 79.52(6)° (Figure 4). The sum of internal angles is that of a planar five-membered ring. An octahedral coordination is formed by three PMe3 ligands in meridional positions and is completed by a hydride ligand in calculated position trans to nitrogen. Two trimethylphosphine groups are in opposite positions, P2-Fe1-P3 ) 149.64(2)°, and one trimethylphosphine group is cis to nitrogen, N1-Fe1-P1 ) 87.25(4)°. The most striking feature
Figure 3. Molecular structure of 2 (ORTEP plot with hydrogen atoms omitted, FeH position indicated).
in the molecular structure of 3 is the methyl substituent (C14) attached to the non-metalated phenyl ring. Table 2 summarizes the structural properties of complexes 1-3. Because of strong trans influence of the aromatic sp2 carbon, the Fe-P bond length, which is trans to the metalated carbon, is clearly longer than the other Fe-P distances in all three structures. They all attain a distorted octahedral geometry, and
C-H ActiVation of Imines by Iron Complexes
Organometallics, Vol. 28, No. 7, 2009 2303
Scheme 2
Figure 4. Molecular structure of 3 (ORTEP plot with hydrogen atoms omitted). Scheme 3 Table 1. Properties of Fe-H Metalacycles 1-3 yield [%]
color
dec [°C]
ν˜ [cm-1]
78 48 74
violet violet violet
